Vigorous research activities are being directed to fuel cells as a next-generation energy source, due to their high generation efficiency and low burden on the environment.
A fuel cell is a power generation device in which hydrogen as a fuel and oxygen are allowed to undergo an electrochemical reaction, from which electric energy is elicited. Depending on the type of electrolyte used, fuel cells are classified into: the Solid Oxide Fuel Cell (SOFC); the Molten Carbonate Fuel Cell (MCFC); the Phosphoric Acid Fuel Cell (PAFC); and the Polymer Electrolyte Fuel Cell (PEFC, including DMFCs in which methanol is used). Among others, PEFCs and DMFCs have operating temperatures as low as about 70 to 90° C. (as compared with other types of fuel cells), and enable a higher-efficiency power generation by about 1 kW in the case of PEFCs, and about several hundred W in the case of DMFCs. Therefore, PEFCs and DMFCs are considered promising for applications such as automobiles, mobile devices, and the like. In particular, DMFCs are small in size, and thus their applications to mobile devices are being vigorously studied.
Hereinafter, with reference to FIG. 4, the structure and principle of a polymer electrolyte fuel cell (PEFC) will be described.
FIG. 4(a) is a perspective view schematically showing the structure of a cell (battery) portion 20, which is a minimum structural unit of a polymer electrolyte fuel cell (PEFC). FIG. 4(b) is a schematic diagram showing the principle behind a PEFC.
As shown in FIG. 4(a), a cell 20 in a fuel cell includes an ion exchange membrane (solid polymer membrane) 11 in the middle, with two electrodes being disposed on its sides: a fuel electrode (hydrogen electrode, anode side) 12 and an air electrode (or oxygen electrode, cathode side) 13. The ion exchange membrane 11 is a membrane for allowing protons (H+) to move from the fuel electrode 12 to the air electrode 13. It is often the case that the ion exchange membrane 11 has electrode catalyst layers 14a and 14b on both sides thereof, and thus the ion exchange membrane 11 and the electrode catalyst layers 14a and 14b are collectively referred to as a membrane electrode assembly (MEA). On the outside of the fuel electrode 12 and the air electrode 13, separators 16a and 16b are provided via gaskets 15a and 15b, respectively. Thus, hydrogen (anode side) moves between the MEA and the separator 16a, and oxygen (cathode side) moves between the MEA 20 and the separator 16b (see FIG. 4(b)). On the surface of the separators 16a and 16b, trenches are formed through which reaction gases of hydrogen and oxygen pass.
As shown in FIG. 4(b), at the anode side, hydrogen (H2) is supplied through the trench in the surface of the separator 16a, and uniformly diffused into the electrode catalyst layer 14a by the action of the fuel electrode 12. On the electrode catalyst layer 14a, H2 becomes H+ through a reaction of formula (1) below, and passes through the ion exchange membrane 11 and moves to the electrode catalyst layer 14b on the cathode side. On the other hand, at the cathode side, oxygen (O2) is supplied through the trench in the surface of the separator 16b, and uniformly diffused into the electrode catalyst layer 14b by the action of the air electrode 13. On the electrode catalyst layer 14b, a reaction of formula (2) below occurs between the O2 having been diffused in this manner and the H+ which has moved through the ion exchange membrane 11 from the anode side, whereby H2O is generated.H2→2H++2e−  formula (1)2H++1/2O2+2e−→H2O  formula (2)
At this time, power generation occurs due to the electrons (e−) which are generated at the anode side. Therefore, the separator is required to efficiently supply reaction gases of oxygen and hydrogen to the electrode catalyst layer 14a. 
Depending on the amount of electric power, a plurality of cells (unit cells) having the above construction may be layered so as to be used in the form of a stack. In this case, the separators will act as partitions between unit cells, and therefore are required to ensure that the gas (hydrogen) from the fuel electrode and the gas (oxygen) from the air electrode will not become mixed in between cells.
From such standpoints, a separator is required to have little gas permeability, a good electrical conductivity, a low contact resistance, a good anticorrosiveness, and so on. In particular, there emerge stronger and stronger anticorrosiveness and electrical conductivity requirements in the recent years. As an evaluation criterion of anticorrosiveness, it is proposed that “no rust should occur even if the separator is immersed in a sulfuric acid solution whose pH is about 1”, and so on.
As a separator material having such characteristics, carbon materials have been used in general. However, carbon materials have poor toughness and are brittle, and therefore are difficult to process, thus resulting in a problem of high processing costs.
Therefore, in the recent years, metal materials, especially stainless steel materials, have been considered as separator materials instead of carbon materials, because metal materials are easy to process and incur low processing costs.
For example, there have been proposed separators in which a plating of a metal film, e.g., platinum or gold, is provided on a stainless steel. Since an oxide coating (passivation film) in which Cr has bonded to oxygen occurs on the surface of the stainless steel, the anticorrosiveness is excellent but the contact resistance is large, so that this cannot be used for a separator material as it is. Therefore, it might be conceivable to coat the surface of the stainless steel with a precious metal such as platinum or gold, which have excellent anticorrosiveness and electrical conductivity. However, the adhesion between a passivation film and a metal film is very poor, thus making it difficult to directly form a film of a precious metal on the surface of stainless steel.
Therefore, a method has been employed which involves completely removing the passivation film via etching or the like, and thereafter optionally forming an underlying plating layer containing a metal such as Ni, and then providing a plating of precious metal. Furthermore, in Patent Document 1, for example, a method of forming an underlying metal layer of a Ta, Zr, or Ti layer for obtaining a high anticorrosiveness is proposed. Moreover, the Applicants of the present application have disclosed in Patent Document 2 that a further improvement in anticorrosiveness is possible by forming an intermediate layer containing oxygen, metal atoms that compose the metal layer, and Fe and Cr which are contained in the steel, between a metal layer of Ta or the like and a steel.
[Patent Document 1] Japanese Laid-Open Patent Publication No. 2001-93538
[Patent Document 2] International Publication No. WO 2006/082734A1